381 



degree of endurance and changes, is found to present, under differ- 

 ences in the circumstances, an almost endless variety. Thus the 

 results, it was found, might be varied by differences in the time of 

 gazing on the object ; by differences in the intensity of the external 

 light, and by the partial or total exclusion of the light of the room 

 from the eyelids ; by alterations in the degree of compression of the 

 eyelids ; by the movement of the eyeballs during the time of obser- 

 ving the picture ; as also by variations in the normal state of the eyes 

 on commencing the experiments. 



All these influencing circumstances had been made successively, 

 or sometimes combinedly, the subjects of special investigation by the 

 author ; and ultimately, in most respects, he considered, so far as 

 his own eyes might be deemed to yield general phenomena, with 

 satisfactory or conclusive results. Various experiments had been 

 made on the spectra derived from light reflected from opake objects 

 in comparison with those elicited by light transmitted by transparent 

 substances, both white and coloured ; as also on the differences in 

 the measure of endurance, the variety of their repetitions, and the 

 phenomena of their changes in colour, of the pictures photographed 

 within the eye, under curiously modified conditions. 



The present communication, however, comprises only a part of 

 these investigations, the first of the cases referred to at the outset, 

 viz. inquiries respecting colourless pictures on the retina, derived 

 from the viewing of objec's under low or moderate degrees of light, 

 or of pictures observed irrespective of chromatic effects. 



1. As to the effect of Time in the viewing of an illuminated object, 

 on the nature and permanency of the picture produced, it was 

 found that, in favourable states of the eyes, a mere momentary 

 glance (such as of a window viewed from the back of a room) was 

 sufficient for producing a distinct negative picture of the illuminated 

 aperture, with the cross bars of the window-frame, which, under 

 certain changes, could be seen ordinarily for about 20 seconds, and 

 under strong light, sometimes for an interval of a minute or two of 

 time, if not more. But the impression from a continuous viewing 

 of a window rather strongly illuminated, for a period of a minute, 

 was very remarkable, the image remaining on the retina whilst the 

 experimenter was breakfasting, and also engaged in writing, so as to 

 be distinctly seen, on slightly closing the eyes, an hour afterwards, 

 and, in another case which he particularly describes, after a lapse of 

 80 minutes. 



2. Experiments on the effects of quantity or intensity of light, on 

 the visual spectra derived from uncoloured objects, showed that 

 such spectra were yielded by extremely low degrees of illumination. 

 The light, for instance, of the moon or stars thrown on a white 

 linen blind, produced distinct negative pictures of the slightly illu- 

 minated aperture. Candlelight gave also negative pictures of white 

 and black objects. Low illumination from transmitted solar light 

 gave, in most cases, colourless pictures, appearing sometimes imme- 

 diately on closing the eyes, as by a flash of light, or otherwise in 3 

 to 5 seconds in negative tints ; these pictures, where the object had 



